Project Abstract. The primary goal of this Career Development Award is the successful transition of the PI from being a mentored to an independent research scientist, studying the immunobiology of diseases of poverty affecting children, such as dengue fever and malaria. The PI and mentoring team have designed a training plan consisting of specific objectives that will provide the PI with practical experience conducting international research, specialized skill using mass cytometry for immunologic research, proficiency in managing complex data, and expertise in the development of adaptive immunity. To achieve these objectives, the team has developed a research plan to investigate the observation that many febrile Kenyan children, with PCR- confirmed dengue virus (DENV) viremia, did not develop serum anti-DENV IgG by 1 month after infection. This antibody hyporesponsiveness to infection was unexpected. Due to the high transmission of Plasmodium falciparum (Pf) malaria in the region, observations of children with simultaneous infection with both DENV and Pf (DENV/Pf co-infection), and published evidence of impaired immunity to Pf mediated by Pf, we hypothesize that during DENV/Pf co-infection, Pf-mediated B cell dysregulation steers antigen-stimulated maturation of DENV-nave B cells toward becoming atypical memory B cells, which are less responsive to stimulation, leading to impaired development of or rapid loss of anti-DENV IgG, particularly lower avidity antibodies. Loss of lower avidity anti-DENV antibodies may affect the risk of antibody-dependent enhancement of DENV infection, which may affect manifestations of clinical disease. To investigate this hypothesis, we propose two aims that will investigate two child acute febrile illness cohorts. In Aim 1, we will investigate the clinical disease spectrum of DENV/Pf co-infected children in relation to parasitemia, viremia, and development of anti-DENV IgG. We will use PCR to detect sub-microscopic Pf parasitemia, which may influence clinical disease or anti-DENV antibody development. We will characterize the clinical disease by infection and develop an acute febrile illness severity score to assess disease severity irrespective of infectious etiology. And we will characterize, longitudinally, the post-infectious development of anti-DENV IgG. In Aim 2, we will profile the memory B cell response after acute DENV solo- or DENV/Pf co- infection. We will use mass cytometry to characterize changes in peripheral B cell populations over time, and perform whole blood stimulation experiments to probe responses of peripheral B cells to antigen exposure. Together, the data collected will answer the question of whether concurrent Pf infection impairs development of anti-DENV antibody responses in an antigen non-specific manner. The results may raise the question of whether and how Pf co-infection might affect vaccine-elicited serum antibody responses, which will provide important considerations in the planning for future deployment of DENV, and possibly other, vaccines.